The answer is Bantu. They had already developed immunity to malaria due to exposure in their native environment. As they migrated southwards to conquer more land, they met the pygmies and the Khoisan. The Bantus had carried with them the malaria vectors. When the mosquitos infected the pygmies and the Khoisan populations, who had no immunity against malaria, the disease reduced their populations drastically hence enabling the Bantus to conquer their land.